(1) Field of the Invention
The present invention relates to landing gear provided with a stiffener in order to increase its stiffness in roll, and to an aircraft having such landing gear. More precisely, the invention lies in the technical field of skid landing gear.
(2) Description of Related Art
Conventionally, a rotorcraft has landing gear on which the aircraft stands when on the ground. More particularly, landing gear includes so-called “skid” landing gear provided with first and second longitudinal bearing skids. The skids are for coming into contact with the ground and they are arranged on either side of the fuselage of the rotorcraft. The first longitudinal bearing skid and the second longitudinal bearing skid together define a plane referred for convenience as the “bearing” plane.
The aircraft then stands on the ground via two elongate skids.
Furthermore, skid landing gear is usually provided with a first cross-member and with a second cross-member, each connecting the first skid to the second skid and serving to connect each of the skids to the fuselage of the aircraft.
The first cross-member is referred to as a “front” cross-member since the first cross-member connects the fuselage to zones situated near the front ends of the first and second longitudinal skids. Conversely, the second cross-member is said to be a “rear” cross-member insofar as the second cross-member connects the fuselage to zones situated near the rear ends of the first and second longitudinal skids.
The landing gear is thus fastened to the aircraft via the front and rear cross-members.
Such landing gear is very effective and enables a rotorcraft to land on numerous types of surface.
A rotorcraft that has at least three hinged blades may be subjected in particular to a phenomenon of ground resonance.
The oscillations of each blade about its lead/lag axis can become coupled in unstable manner with movements of the fuselage of the rotorcraft depending on the elastic deformation modes of the landing gear: this is at the origin of the so-called “ground resonance” phenomenon.
As they rotate, the blades move away from their equilibrium positions and can thus become distributed non-uniformly. This non-uniform distribution gives rise to unbalance by moving the center of gravity of the rotors away from the axis of rotation of the rotor. Furthermore, blades that are offset from their equilibrium positions oscillate about those equilibrium positions at an oscillation frequency ωδ, which is the resonant frequency of the blades in lag.
If Ω is the frequency of rotation of the rotor, the fuselage of the rotorcraft is thus excited at two frequencies |Ω±ωδ|.
When standing on the ground on the landing gear, the rotorcraft fuselage may be thought of as a mass system that is supported above the ground by a spring and a damper constituted by the downward branches of the cross-members. The fuselage supported in this way on the landing gear is thus characterized by resonant modes of vibration in roll and in pitching. There is a danger of instability on the ground when the resonant frequency of the fuselage in roll or in pitching comes close to the resonant frequency of oscillation |±ωδ| or |Ω−ωδ|, and this corresponds to the ground resonance phenomenon.
In order to avoid such instability, it is known in particular to seek to avoid those frequencies crossing by adapting the stiffness of the landing gear in roll and/or pitching.
Nevertheless, it can be complex to adapt landing gear. A compromise needs to be found in particular between firstly the vertical stiffness of the landing gear, which determines comfort and also the loading imparted to the structure during a landing, and secondly the stiffnesses in pitching and in roll that have a large influence on the ground resonance behavior.
It is recalled that the person skilled in the art uses the term “vertical stiffness” to designate the stiffness of the landing gear under the effect of gravity along the elevation axis of the aircraft, and ignoring its static trim.
Developing skid landing gear is thus a process that is generally lengthy and difficult. This development is therefore rarely reviewed during the lifetime of an aircraft.
However, substantial modifications may be made to an aircraft during its lifetime. The resonant frequencies of the fuselage in roll and/or in pitching can therefore change, and thus run the risk of causing the ground resonance phenomenon to appear.
Under such circumstances, a manufacturer may attempt to modify the stiffnesses of the landing gear in roll and in pitching, without having too much influence on the behavior of the aircraft, in particular when landing.
For this purpose, shape modifications may be applied to skid landing gear. Nevertheless, such shape modifications can present the drawback of modifying the vertical stiffness of the landing gear. In particular, this can have an impact on the behavior of the landing gear.
Certification tests may then be required in order to demonstrate that the modified landing gear still complies with the certification regulations that are in force.
It should not be forgotten that developing skid landing gear is a process that can be lengthy and expensive.
An object of the present invention is thus to propose landing gear fitted with a stiffener, and also to propose an aircraft including such landing gear.
In this context, the technological background includes document FR 2 554 210 which describes a flexible beam made of composite materials and substantially in the form of an elongate box of laminated structure. Two rigid sole plates are connected together by two webs.
An energy-absorbing deformable stud is placed between the two sole plates and includes at least one block of elastomer material having a high level of remanence in deformation.
The beam also includes at least one viscoelastic damper mounted on the outside face of the sole plate. The damper is stressed in traction by the bending deformation of the beam so as to produce damping that is added to the damping provided by each energy absorber stud.
Such a beam may be used on skid landing gear.
That configuration suggests using an elastomer within a cross-member and within a damper fastened under the cross-member. That configuration is remote from the invention and it has little influence on the location of the resonant frequencies of the fuselage in roll or in pitching relative to the resonant frequencies of oscillation |Ω±ωδ| or |Ω−ωδ|.
Document U.S. Pat. No. 4,270,711 describes landing gear provided with a beam that is connected by a pivot to a cross-member of the landing gear so as to be capable of pivoting about a pivot axis. The ends of the beam are then fastened to the structure of the aircraft.
That teaching makes it possible in particular to fasten landing gear having at least three fastener points to a structure that has four fastener points.
Document U.S. Pat. No. 6,244,538 describes landing gear.
That document makes it possible to locate the resonant frequencies of the fuselage in roll or in pitching relative to the resonant frequencies of oscillation |Ω±ωδ| or |Ω−ωδ| as a function of the flare point of support connecting rods.
Document U.S. Pat. No. 3,173,632 describes landing gear having two skids connected by two torsion rods. Each torsion rod is secured to two arms, with each arm being hinged to an upright extending upwards from a skid.
Movement prevention means are also provided for authorizing or preventing each torsion rod to move in pivoting about its axis of symmetry.
Also known is Document U.S. Pat. No. 4,519,559.